1. Field of the Invention
This invention relates generally to a carbon brush for miniature motors and the method of making the same, and more particularly to a carbon brush for miniature motors used in a miniature motor having a permanent magnet field, which a metal-plated graphite brush formed by coating particles of graphite powder with a metallic layer, and then pressure-forming and sintering the graphite powder; the ash content of the graphite powder being reduced to less than 0.05 wt. % to reduce mechanical noise and improve commutation properties.
2. Description of the Prior Art
Carbon brushes for miniature motors have heretofore been manufactured by adding a binder to graphite powder purified to approximately to 99% or 99.5%, grinding and screening the solidified mixture, blending metallic powder with the ground and screened mixture to impart desired electrical conductivity as necessary, and then pressure-forming and sintering the resulting mixture.
To eliminate the use of the binder, a so-called copper-plated graphite brush is known. The copper-plated graphite brush is manufactured by copper-plating particles of graphite powder which is purified to approximately 99%, then pressure-forming and sintering the copper-plated graphite powder without adding a binder.
The former process of the above-mentioned conventional methods involves the forming and sintering of graphite powder (containing ashes) together with a binder. The use of the binder produces the residual carbon formed as the result of the sintering and carbonization of the binder, causing the composition strength to increase. The increased composition strength tends to increase mechanical noise generated when the brush thus manufactured makes sliding contact with the surface of the commutator.
The latter process of manufacturing a copper-plated graphite brush, on the other hand, involves application of a copper layer onto the surface of the particles of graphite powder. In this process, the copper-plated graphite powder is pressure-formed and sintered without using a binder. The absence of the binder with the copper-plated graphite brush helps reduce mechanical noise, compared with the carbon brush manufactured using the binder.
The copper-plated graphite brush, however, has insufficient commutation properties because of the presence of the ash content in the graphite powder.
The conventional copper-plated graphite brush has a number of unwanted problems. For example, the ash particles when brought in contact with the commutator surface, tend to produce scratches on the commutator surface, causing sparks to generate during the subsequent commutation. In addition, the presence of the ash particles also cause instantaneous conduction failure.